The present invention is related to the field of silica crucibles, and more specifically to a silica crucible having a multi-layer wall in which one or more of the wall layers are doped with barium.
The Czochralski (CZ) process is well-known in the art for production of ingots of single crystalline silicon, from which silicon wafers are made for use in the semiconductor industry.
In a CZ process, metallic silicon is charged in a silica glass crucible housed within a susceptor. The charge is then heated by a heater surrounding the susceptor to melt the charged silicon. A single silicon crystal is pulled from the silicon melt at or near the melting temperature of silicon.
A current trend in the semiconductor industry is toward large diameter wafers, e.g., 300-400 mm in diameter. As a result, the CZ process operating period must be concomitantly increased, sometimes to more than one hundred hours. Also, structural defects in the silicon crystal can be reduced by slowing down the pulling rate, which in turn prolongs the CZ run time and emphasizes the need to improve the useful life of the crucible.
At operating temperatures, the inner surface of a silica crucible frequently reacts with the silicon melt. In many cases, the inner surface of the crucible undergoes a change in morphology. The inner surface of a crucible is seen to roughen during prolonged operation in a CZ run. This roughening, and the phase transformation underlying it, are addressed in greater detail below.
This roughening can cause a loss of crystal structure of the pulled ingot. Inner surface roughening renders the crucible unfit for use in silicon ingot manufacture. When a major portion of the inner surface of the crucible is covered by a rough surface, crystalline structure is disrupted at the crystal-melt interface. Such a roughened crucible is unsuitable for ingot manufacture and silicon crystal pulling using a roughened crucible must be ceased to avoid manufacture of substandard ingots.
Additionally, the inner surface of a silica glass crucible can partially dissolve into the silicon melt during the CZ process. Silicon and oxygen, the main components of a silica crucible, are not deleterious to the silicon melt. However, impurities in the inner layer of the crucible can be transferred to the silicon melt during this process. The quality of the pulled single crystal may be ruined, depending on the extent of contamination and the nature of the contaminant.
One such effort to control inner surface morphology is a crucible with barium-containing chemicals coated onto the inner surface. U.S. Pat. Nos. 5,976,247 and 5,980,629, both to Hansen et al., disclose a crucible incorporating a devitrification promoter on the inner surface of the crucible. The devitrification promoter is taught to prevent particulate generation at the silica-melt interface. The devitrified layer, created during a CZ run, is described in these references as a crystallized silica layer and is reported to dissolve uniformly and maintain a smooth crucible inner surface.
Barium carbonate (BaCO3) is disclosed as a preferred coating material, although other alkaline-earth metal compounds are disclosed. Coating is done as a post-treatment of a finished crucible by applying a solution of barium-containing chemicals. The coated crucible is then dried using clean, hot air.
If the crystalline layer thickness exceeds a certain level, the crucible is prone to cracks and possible leakage of the silicon melt. Despite careful optimization of the barium coating level, the crucible nevertheless occasionally experiences cracking toward the end of a CZ run.
However, devitrification (i.e., crystallization) occurs in a shallow layer on the inner surface of the crucible. The silica glass so coated experiences a large volume change as it crystallizes when barium coating is used as a crystallization promoter. The volume change creates stress at the glassy phase-crystalline phase interfaces. Such stress is relieved by micro-scale deformation in the glassy phase of the crucible.
Other drawbacks to barium coating include difficulty in controlling the thickness and uniformity of barium per unit area on the crucible surface. The drying procedure is also prone to introduce airborne contamination.
Additionally, BaCO3 is poorly soluble in water, but the coating can be easily removed by rinsing or wiping the inner surface with water. Normal cleaning procedures (e.g., rinsing, etching or wiping) cannot be performed after barium coating. Crucibles must also be carefully stored until used.
One of the present inventors filed Japanese Patent 3100836 (laid open Tokukai Hei8-2932), which teaches an inner layer of 0.5-1 mm in thickness and containing from 0.1-2% aluminum by weight. The inner layer crystallizes during the CZ process, such that inner surface dissolution is suppressed and the dimensional stability of the crucible is improved.
However, aluminum may dissolve into the silicon melt and subsequently lodge in the silicon crystal. The level of aluminum contamination of the crystal can be successfully controlled in some cases. Nevertheless, there are applications wherein aluminum contamination is undesirable.
The present disclosure provides a silica glass crucible comprising a wall with a barium-doped layer formed as an integral part of the crucible. The inner layer is doped with barium at a concentration such that the inner layer will rapidly crystallize upon heating. Utilization of a doped layer, rather than a coating on the interior surface, permits the crucible to be handled and processed without damage to the barium-doped inner layer.
A method is disclosed for making a silica crucible having an inner layer doped so as to devitrify during a CZ run. The method comprises introducing into a rotating crucible mold bulk silica grain, consisting essentially of quartz grain, to form a bulky wall. After heating the interior of the mold to fuse the bulk silica grains, an inner silica grain, doped with barium, is introduced into the mold. The heat also at least partially melts the inner silica grain, allowing it to fuse to the wall to form an inner layer. The crucible thus formed is cooled then taken out of the mold.
The invention will become more readily apparent from the following detailed description, which proceeds with reference to the drawings, in which: